Fabric of Geology

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UNDER CONSTRUCTION…

Draft – Subject to Revision

Laws, Principles, and Doctrines of Geology

1) Uniformitarianism

2) Superposition

3) Faunal Succession

4) Original Horizontality

5) Cross-cutting Relationships

6) Inclusions

7) Unconformities

8) Walther’s Law

Laws, Principles, and Doctrines of Geology

“Nature does not reveal all her secrets at once.  We imagine we are initiated in her mysteries: we are as yet, but hanging around her outer courts.”

[  Seneca  ]

1) Uniformitarianism – One of the most basic of all geological principles, uniformitarianism is commonly paraphrased “the present is the key to the past”.  Keep in mind – the basis of geological thought is the rock, that is, any rock upon which we may wish to interpret an origin.  How did large bodies of rock that form the structural foundation of the earth come to be?  Where do the rocks that make up a volcano, a mountain range, the sides of a vast canyon, or a rocky coastline come from in the first place?   Nature presents us with three types of rocks – (1) igneous, or fire-formed, (2) sedimentary, meaning the rock is made up of particles and/or crystals derived from earth-surface processes, and (3) metamophic, or “changed” rock.  This latter group of rock, metamorphic, was either an igneous or sedimentary rock to begin with but has undergone changes in appearance through “baking” at elevated temperatures and pressures within the earth.

The doctrine of uniformitarianism holds that the origin of ancient rocks we observe in nature can be understood by studying how rocks are forming today.  Uniformitarianism provides a context within which to interpret how rocks have come to be – and that context is the present-day world we see out the window.  Where on the landscape are we forming igneous, sedimentary, and metamorphic rocks today?  This question is easy to answer for the first two rock types – igneous and sedimentary – because these two rock types are forming today, right before our eyes.

Igneous rocks are forming today in volcanoes, or more obviously, volcanoes are made up of different igneous rock types.  One type of igneous rock, basalt, forms from congealed molten liquid (lava) that has accumulated and flowed at or near the surface of the earth.  As a result, portions of our landscape are covered with ancient lava flows made of the igneous rock basalt.  In order to understand how ancient lava flows of basalt may have formed in the past, such as the flows we see today at Black Mesa in the Oklahoma panhandle (180 million years old), on the Isles of Mull and Iona (50-60 million years old) off the coast of Scotland, or in Yellowstone National Park (flows ranging in age from 60,000 to 600,000 years old), we travel to and study igneous rocks at locations where lava flows are forming today.  Three of the best places to study the formation of lava flows are Hawaii (watch these personal recollections), Iceland (where a major eruption occurs every four years on average), and the Democratic Republic of Congo (where many residents of the town of Goma are directly impacted by lava flows).

Likewise, sedimentary rocks are also forming on the surface of the earth today.  The two major types of sedimentary rocks are (1) clastic and (2) chemical.  Clastic sedimentary rocks are made up of particles of pre-existing rocks and soils that have been physically transported by water, wind, or ice.  The particles, derived through weathering and erosion of the landscape, can range in size from boulders to gravel, sand, silt, and clay.  For the purpose of showcasing how to apply the concept of uniformitarianism to clastic sedimentary rocks, a comparison is made between the appearance and processes associated with accumulation of modern river deposits as a means to infer the origin of sedimentary rocks formed by ancient rivers.

A significant volume of sediment is transported by major rivers to coastal areas each year.  Erosion of the landscape and the volume of sediment in transport by rivers is substantially greater during and immediately following large rainfall-runoff events.  Because large rainfall events are intermittent, movement of sediment down the rivers to the coast is intermittent.  When not in transport by rivers, sediment is “stored” intermittently at various locations along the river valley enroute to the seacoast.  A portion of the sediment transported down a river is stored in the river floodplain, the broad flat areas adjacent and parallel to the main river channel.  Some of the river sediment is stored also in the bed and banks of the channel.  Local river currents associated with flow in the active channel and waves generated by the wind sculpt the bed and bank sediments into sand bars with a host of characteristic features that help to distinguish the sand bars as products of river processes.

Some of the the sediment transported to coastal areas by rivers is stored as layers of sediment in estuaries, bays, and deltas.  Sediment, mainly sand, introduced by rivers to the parts of the coast that face the ocean are stored in beaches, barrier islands, and spits.  These latter geomorphic features develop in response to current and wave activity in the coastal zone driven by both water and wind energy.  Some silt and clay-size particles are stored in the marshes, behind the barrier islands.  During significant river discharge events, silt and clay-size particles held in suspension by sea currents are carried further oceanward to settle and form “blankets” of layered sediment on the continental shelf.  Some particles are transported over the edge of the continental shelf where they slowly settle through the water to form spatially extensive layers of sediment on the continental slope, continental rise, and in the ocean-basin deeps.

Therefore, when we see an exposure of ancient rocks along the road or in a canyon wall, we form a mental picture about the origin of the rocks based on comparison to locations on the earth today where similar types of rocks are forming today.  In this sense, the present is a key to the past!

Or is it?

 

 

2) Superposition

3) Faunal Succession

4) Original Horizontality

5) Cross-cutting Relationships

6) Inclusions

7) Unconformities

8) Walther’s Law

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UNDER CONSTRUCTION…

Draft – Subject to Revision

 

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